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Báo cáo khoa học: "A SNoW based Supertagger with Application to NP Chunking"

Supertagging is the tagging process of assigning the correct elementary tree of LTAG, or the correct supertag, to each word of an input sentence1 . In this paper we propose to use supertags to expose syntactic dependencies which are unavailable with POS tags. We first propose a novel method of applying Sparse Network of Winnow (SNoW) to sequential models. Then we use it to construct a supertagger that uses long distance syntactical dependencies, and the supertagger achieves an . | A SNoW based Supertagger with Application to NP Chunking Libin Shen and Aravind K. Joshi Department of Computer and Information Science University of Pennsylvania Philadelphia PA 19104 USA libin joshi @linc.cis.upenn.edu Abstract Supertagging is the tagging process of assigning the correct elementary tree of LTAG or the correct supertag to each word of an input sentence1 . In this paper we propose to use supertags to expose syntactic dependencies which are unavailable with POS tags. We first propose a novel method of applying Sparse Network of Winnow SNoW to sequential models. Then we use it to construct a supertagger that uses long distance syntactical dependencies and the supertagger achieves an accuracy of 92.41 . We apply the supertagger to NP chunking. The use of supertags in NP chunking gives rise to almost 1 absolute increase from 92.03 to 92.95 in F-score under Transformation Based Learning TBL frame. The surpertagger described here provides an effective and efficient way to exploit syntactic information. 1 Introduction In Lexicalized Tree-Adjoining Grammar LTAG Joshi and Schabes 1997 XTAG-Group 2001 each word in a sentence is associated with an elementary tree or a supertag Joshi and Srinivas 1994 . Supertagging is the process of assigning the correct supertag to each word of an input sentence. The following two facts make supertagging attractive. Firstly supertags encode much more syntactical information than POS tags which makes supertagging a useful pre-parsing tool so-called almost parsing Srinivas and Joshi 1999 . On the 1By the correct supertag we mean the supertag that an LTAG parser would assign to a word in a sentence. other hand as the term supertagging suggests the time complexity of supertagging is similar to that of POS tagging which is linear in the length of the input sentence. In this paper we will focus on the NP chunking task and use it as an application of supertagging. Abney 1991 proposed a two-phase parsing model which includes .